perfSONAR User Q&A and Other Discussion

[Zhi-Wei Lu <>]

Hi Shawn,

 

Icmp unreachable was blocked on our routers, which caused pmtu discovery to fail one way (while the other way PMTU was found to be 1500), thus application use MTU=9000 and IP no fragment was set, application fails.

 

Thanks again Shawn.

 

Zhi-Wei Lu

IET-CR-Network Operations Center

University of California, Davis

(530) 752-0155

 

From: Shawn McKee [mailto:]
Sent: Wednesday, April 16, 2014 3:31 AM
To: Zhi-Wei Lu
Cc: ; Kevin Kawaguchi
Subject: Re: [perfsonar-user] MTU=9000 and perfSonar bandwidth test issue

 

Hi  Zhi-Wei,

 

Can  you try 'tracepath' from your  perfSONAR host  to  nersc:

 

tracepath  perfsonar.nersc.gov

 

The  problem you are  hitting  could be  that  some  device on  the  path  is  blocking  the  needed  ICMP  packets  for  PMTUD  (Path  MTU  Discovery)  to  work.     

 

Perhaps others  on  the list  have  suggestions  for finding  the source  of  the  problem.

 

Shawn

 

On Tue, Apr 15, 2014 at 7:52 PM, Zhi-Wei Lu <> wrote:

Hi all,

 

We are experiencing some strange issue (ip fragmentation) for our perfSonar bandwidth test with MTU=9000.  For perfSonar machines with clean MTU=9000 path,  our band width tests between the pair of perfSonar boxes work fine.  However, if the path between the two perfSonar boxes have MTU=1500 devices (such as firewalls or packet shape), only one way test would work.

 

For example, from our box p0 (bebind 1500 MTU security device) to perfsonar.nersc.gov

bwctl –c perfsonar.nersc.gov  would fail, while

bwctl –s perfsonar.nersc.gov would work

 

From our box pb (on the border) with a clean 9K MTU path to perfsonar.nersc.gov, both tests work.

Test between out boxes pb and p0 would exhibit the one way behavior as well, as there is MTU=1500 device in the path.

 

I though that IP fragmentation is enabled on the Linux boxes (CentOS 6.5), yet why I am getting this one way traffic behavior? Are there kernel/application parameters that I can set to correct the issue that we are encountering.

 

If I set MTU=1500 on p0, tests to any site work both ways.  Thank you.

 

Zhi-Wei Lu

IET-CR-Network Operations Center

University of California, Davis

(530) 752-0155